1. Field of the Invention
This invention relates generally to optical communication systems and more particularly to optical laser sources with multiple lasing wavelengths.
2. Prior Art of the Invention
Dense wavelength division multiplexing (DWDM) is a very important technology for fiber optical communications, since it provides a very economical and efficient way to increase the transmission capacity by exploiting the enormous bandwidth available in optical fibers. In this technology the bandwidth in the low attenuation band of the optical fiber is subdivided into a number of wavelength channels where each channel carries data up to a maximum rate which is accessible to electronic interfaces.
At the transmitter side of DWDM networks, Multi-Wavelength Laser Sources (MWLS) are required to enable parallel transmission on a large number of wavelength channels. On the other hand, MWLS are also required to test DWDM components, modules and systems. This has been one of the most troubling shortcomings in test and measurement technology, since single channel scanning methods are used to characterize such systems and components. However, a true test for a DWDM module or component must include all the channels at the same time. MWLS can provide an efficient means to perform DWDM testing.
A straightforward approach to realize MWLS is to combine the output power from an array of single wavelength lasers into a single fiber [M. G. Young, U. Koren, B. I. Miller, M. A. Newkirk, M. Chien, M. Zirngibl, C. Dragone, B. Tell, H. M. Presby, and G. Raybon, “A 16×1 wavelength division multiplexer with integrated distributed Bragg reflector lasers and electroabsorption modulators,” IEEE Photon. Technol Lett., vol. 5, pp908-910, 1993].
Channel spacing regularity can be improved using multiple wavelength semiconductor lasers based on an array of physically separated gain media with lasing wavelengths determined by diffraction gratings; [M. C. Farries, A. C. Carter, G. G. Jones, and I. Bennion, “Tunable multiwavelength laser with single fiber output,” Electron. Lett., vol. 27, pp1498-1499, 1991], or waveguide grating routers; [M. Zirngibl, C. H. joyner, C. R. Doerr, L. W. Stulz, and H. M. Presby, “An 18-channel multifrequency laser,” IEEE Photon. Technol. Lett., vol. 8 pp870-872, 1996].
Erbium doped fiber (EDF) lasers have been attracting a lot of research and applications because of their high gain and efficient oscillation. However, it is not easy to obtain a multi-wavelength laser output from EDF lasers. In an EDF laser, all the lasing modes of the channels compete with each other. Only the laser oscillating modes with gain higher than the threshold can exist in the laser cavity and emit from the cavity. In order to realize multi-wavelength laser output from EDF lasers, the competition between laser channel modes must be reduced and the gain must be equalized for all channels. At room temperature, the homogenous line broadening of the gain spectrum is predominant. This effect must be suppressed, otherwise, the gain will be clamped by the resonator loss at only one lasing frequency and separate gain media have to be used for each wavelength channel. One method is to cool EDF lasers in liquid nitrogen (77° K) to reduce the homogenous broadening and cross-gain saturation effects in the fiber; [S. Yamashita and K. Hotate, “Multiwavelength erbium-doped fiber laser using intracavity etalon and cooled by liquid nitrogen,” Electron. Lett., vol 32, pp1298-1299,1996]. The other method is to use a frequency shifter to promote multi-wavelength operation, [H. Sasamori, M. Isshiki, H. Watanabe, and K. Kasahara, “Multi-wavelength erbium-doped fiber ring light source with fiber grating filter,” in Proc. Tech. Dig. Optical Amplifiers Applications (OAA '97), 1997, Paper WC3, pp235-238], [A. Bellemare, M. Rochette, M.Tetu, and S. LaRochelle, “Multifrequency erbium-doped fiber ring lasers anchored on the ITU frequency grid,” in Proc. Tech. Dig. Optical Fiber Commun. Conf. (OFC'99), 1999, Paper TuB5, pp. 16-18].
The present invention provides, a novel design based on EDF ring lasers, which simultaneously provide a number of wavelength channels.